1. Field of the Invention
The present invention relates to a piezoelectric actuator, and, more particularly, to an improvement in a d.sub.31 type layered piezoelectric actuator which generates a large displacement.
2. Description of the Related Art
A conventional d.sub.31 type layered piezoelectric actuator 1, which is related to the present invention, is shown in FIG. 8. The d.sub.31 type layered piezoelectric actuator 1 is planar and substantially rectangular. One end of the actuator 1 is fixed to a fixing portion 2 such as a base and the other end of the actuator 1 is unsupported and free to experience displacement.
The piezoelectric actuator 1 includes an actuator body 6 having a layered structure including a plurality of piezoelectric layers 3 and a plurality of inner electrodes 4 and 5 that are disposed between the piezoelectric layers 3. The actuator body 6 has a first major surface 7 and a second major surface 8, as well as, a first end surface 9 and a second end surface 10. The first and second major surfaces 7 and 8 are defined by the outwardly facing major surfaces of the two piezoelectric layers 3a and 3b that form the outermost layers of the actuator body 1. The end surfaces 9 and 10 are defined by the end surfaces of the plurality of piezoelectric layers 3.
A first external electrode 11 and a second external electrode 12 are disposed on the first end surface 9 and the second end surface 10 of the actuator body 6, respectively. In order to actuate the piezoelectric actuator, a voltage is applied to the first and the second external electrodes 11 and 12 from outside of the actuator body 6.
The aforementioned inner electrodes 4 are referred to as first inner electrodes 4 and are connected to the first external electrode 11 on the first end surface 9. The inner electrodes 5 are referred to as second inner electrodes 5 and are connected to the second external electrode 12 on the second end surface 10. Applying a voltage to the external electrodes 11 and 12 causes the first inner electrodes 4 and the second inner electrodes 5 to have different polarities. The first inner electrodes 4 and the second inner electrodes 5 are alternately disposed in the direction in which they are stacked upon each other.
At the side of the first end surface 9 of the actuator body 6, a piezoelectrically active area 13 is defined by stacking the first inner electrodes 4 and the second inner electrodes 5 upon each other. The piezoelectrically active area 13 is located closer to the free end of the actuator 1 than the fixed end of the actuator attached to the fixing portion 2. At the side of the second end surface 10 of the actuator body 6, a piezoelectrically inactive area is defined and has a size or area which is equal to the fixing area to be fixed to the fixing portion 2. The fixing area is fixed so that the piezoelectric actuator 1 is not dislodged or removed from the fixing portion 2 when the actuator body 6 is undergoing displacement.
For the purpose of increasing displacement, the piezoelectric layers 3a and 3b, being the outer most layers of the illustrated piezoelectric actuator 1, are formed so as to be piezoelectrically active within at least the piezoelectric active area 13. More specifically, the first external electrode 11 has an extension portion 14 and an extension portion 15 which extend to the first major surface 7 and the second major surface 8, respectively, with the extension portion 14 facing, through the piezoelectric layer 3a at the first major surface 7 side, the second inner electrodes 5 having a polarity which is different from that of the first outer electrode 11, and the extension portion 15 facing, through the piezoelectric layer 3b at the second major surface 8 side, the second inner electrodes 15.
The displacement of the piezoelectric actuator 1 is shown in FIG. 9. In FIG. 9, the piezoelectric actuator 1 is deformed to the shape indicated by broken lines, causing displacement shown by arrow 16 at one end portion of the piezoelectric actuator 1.
The above-described piezoelectric actuator, though displaced by only a small amount, is displaced easily and quickly. As a result, the actuator can be advantageously used in inkjet printers. However, when a high-performance printer is to be provided, it is necessary to discharge ink in large amounts. In such a case, an actuator which can is arranged to experience a large displacement is desired.
To achieve such an actuator, it has been attempted to develop a new material having a large d.sub.31 constant to produce a highly efficient d.sub.31 type actuator. However, even this piezoelectric actuator does not provide sufficient performance or large enough displacement. In addition, in developing the new material having a large d.sub.31, considerable effort and time are required to increase the d.sub.31 constant by only a few percent.